We see two red wires in this photo.
The one seperate from the harness looks to be larger than 12 gage ? Maybe as large as 8 gage?
View attachment 1715886155
Is that bigger wire the one connecting here?
View attachment 1715886176
10 gage to the fusible link was standard in '73
I ask because some cars got a heavy duty alternator and battery wiring.
One version came on cars equipped with rear window electric defrosting grids.
Another version was the police/taxi fleet wiring - very rare on A-bodies since most weren't severe use fleet cars. These options used 8 gage wire.
One clue will be if the rear window has a defroster grid.
Another will be if there is a grommet in firewall for those wires.
There's a photo of the firewall grommet feeds here:
How rare is electric rear defrost?
and another version here
Optional Heavy Duty wiring
Neither should effect your trouble shooting method. But they will change your wiring diagram a little.
